(Adapted from the Applicant's Abstract.) The goal of this proposal is to determine the structural organization of central auditory pathways that process binaural information. The functional types of afferents that project to the central nucleus of the inferior colliculus are quite diverse and it is possible that single IC neurons integrate information from many afferent sources. At present there is insufficient information about the organization of these afferents to determine their contribution to receptive field properties and other response characteristics of binaural cells. One hypothesis is that axonal distribution and morphology are heterogeneous within iso-frequency planes. Thus, many different combinations of afferents would converge in synaptic zones and contribute to response properties of cells in those zones. A second hypothesis is that relative spatial relationships in iso-frequency regions of afferent nuclei are preserved by the organization of the projections to the inferior colliculus. The topology would provide a place code for binaural features. To address questions regarding the two hypotheses, a series of experiments detailing the organization of the afferent connections to the inferior colliculus in the adult cat are proposed. Specific aims are: 1) to define the geometry of the afferent sheets and single axons of medial superior olivary projections to the IC using bulk and single axonal anterograde tracing methods; 2) to compare the geometry of converging axonal systems with the organization of the MSO axons with methods to co-label projections from more than one source in the same sections; and 3) to relate the distribution and morphology of intra-axonally labeled axons to response characteristics of electrophysiologically identified cells sending axons to IC. The results will lead to further hypotheses about the organization of axonal interactions and their relationship to binaural processing in the IC. Binaural processing is important for sound localization, coding of complex features of speech sounds and for phenomena such as masking and signal detection. Improving assessment of these functions in hearing impaired individuals requires experiments such as the one proposed to increase our knowledge about the anatomical and physiological basis for neural processing of sound.